Some ideas for altars dedicated to a certain element.
Different types of waters (Ocean, River, Rain, Moon water, Sun water, Rose water) Coral, Sea shells, Starfish, Fish bones, Pearls, Chalice, Silver (Metal)
Colors: Silver, Blue, White, Green, Cream
Deities/Entities: Poseidon, Ceto, Bairthy, Sea & River Nymphs, Thalassa, Amphitrite, Neptune, etc.
Attributes: Fertility, Mystery, Moon-Driven Cycles (Tides & Menstruation), Illusion, Emotions, Feminine Energies, Romance
Candles, Lamps, Resin burners, Salt Lamps, Carnelian, Garnet, Animal Horns, Athame, Bronze, Gold, Amber, Sun Catchers
Colors: Red, Gold, White, Dark Brown, Tan
Deities/Entities: Ra, Brigit, Aed, Logi, Hestia, Hephaestus, Loki, Ares, etc.
Attributes: Passion, Sex, Freedom, War, Destruction & creation, The Sun, Warmth, Confidence
Potted plants, Stones, Wooden & Ceramic bowls, Sand (for incense burning), Petrified wood, Altar tile, Animal hides, Iron, Jasper
Colors: Brown, Black, Tan, Green
Deities/Entities: Gaia, Geb, Danu, Babalú-Ayé, Akka, Demeter, Terra, Sif, etc.
Attributes: Balance, Nurturing, Fertility, Abundance, Parenthood, Protection
Feathers, Incense, Oil burners, Fans, Smoke Bundles, Swords (or Wands depending on your correspondences), Staffs, Flags, Clear Quartz
Colors: Yellow, White, Sheer
Deities: Amun, Hutchai, Shu, Dogoda, Sidhe, Odin, Njord, Aura, Aurai, Boreas, La'a Maomao, etc.
Attributes: Swiftness, Action, Communication, Change, Intelligence, Adaptability